The present invention relates to uniform inorganic microspheres and to a method of producing the same.
A method of producing inorganic microspheres is disclosed in Japanese Patent Publication No. 54-6251 and No. 57-55454.
Said method comprises mixing an aqueous solution of an inorganic compound with an organic solvent to give a w/o type emulsion and causing precipitation of emulsion particles formed in said emulsion to thereby obtain inorganic microspheres.
However, it is difficult to produce inorganic microspheres substantially uniform in particle size by the above method. The inorganic microspheres obtained by the above method are not uniform in particle size.
Therefore, to overcome the above disadvantage, a method of producing inorganic microspheres which comprises using the membrane-emulsified reversed micelle technique has been proposed (Japanese Kokai Patent Publication No. 04-154605 (Japanese Patent Application No. 02-277507)).
This method comprises injecting under pressure an aqueous solution of an inorganic compound into an organic solvent through a microporous membrane having a hydrophobic surface. The microspheres obtained by this method have a somewhat narrower particle size distribution.
Nevertheless, the above production method using the membrane-emulsified reversed micelle technique has the following disadvantages.
1) The pores in the microporous membrane are winding and twisty within the membrane and, therefore, the particle source material-containing aqueous solution travels several centimeters through the pores from its entering the microporous membrane to its leaving the same. This causes a great pressure loss in the microporous membrane. In particular when inorganic microspheres having a small particle size or when an aqueous starting material solution having a high viscosity is used, the production per unit time becomes very low.
2) The standard deviation, on the volume basis, for the particle size distribution of the particles obtained is much narrower as compared with the method not relying on the membrane-emulsified reverse micelle technique but still corresponds to 20-50% of the mean particle size. Such variability, which is due to the variability in the micropore size in the microporous membrane and the variability in the shape of the pore opening edge on the membrane surface, is still great and unsatisfactory.
3) The alkali contained in the aqueous source material solution damages the microporous membrane, for example causes peeling of the hydrophobic layer provided on said microporous membrane, hence the service life of said membrane is as short as several hours to scores of hours.
Another known method of producing inorganic microspheres is the so-called sol-gel method, as described in E. Barringer et al.: Ultrastructure Processing of Ceramics, Glasses, and Composites, John Wiley & Sons, New York (1984), pages 315-333.
The method comprises injecting a sol containing a particle source material before polymerization into an organic solvent to form sol emulsion particles and then promoting the polymerization reaction to thereby cause gelation and precipitation of the sol emulsion particles.
In this method, too, the variability in the particle size of the inorganic microspheres obtained is great and this offers a problem.